NASA Sees the Dark Side of the Sun

January
23, 2009: Today, NASA researchers announced an event
that will transform our view of the Sun and, in the process,
super-charge the field of solar physics for many years to
come.

"On
February 6, 2011," says Chris St. Cyr of the Goddard
Space Flight Center, "Super Bowl XLV will be played in
Arlington, Texas."

Wait
… that's not it.

"And
on the same day," he adds, "NASA's two STEREO spacecraft
will be 180 degrees apart and will image the entire Sun for
the first time in history."

Right:
An artist's concept of one of the STEREO spacecraft. [Larger
image]

STEREO's
deployment on opposite sides of the Sun solves a problem that
has vexed astronomers for centuries: At any given moment they
can see only half of the stellar surface. The Sun spins on
its axis once every 25 days, so over the course of a month
the whole Sun does turn to face Earth, but a month is not
nearly fast enough to keep track of events. Sunspots can materialize,
explode, and regroup in a matter of days; coronal holes open
and close; magnetic filaments stretch tight and—snap!—they
explode, hurling clouds of hot gas into the solar system.
Fully half of this action is hidden from view, a fact which
places space weather forecasters in an awkward position. How
can you anticipate storms when you can't see them coming?
Likewise researchers cannot track the long-term evolution
of sunspots or the dynamics of magnetic filaments because
they keep ducking over the horizon at inconvenient times.
STEREO's global view will put an end to these difficulties.

The
global view is still two years away. Already, however, the two
spacecraft are beaming back over-the-horizon images that have
researchers and forecasters glued to their monitors.

"This
is a perspective we've never had before," says STEREO
mission scientist Lika Guhathakurta of NASA headquarters.
"We're now monitoring more than 270 degrees of solar
longitude—that's 3/4ths of the star."

"After
all these years," she laughs, "we're finally getting
to see the dark side of the Sun."

(Editor's
note: The Sun has no dark side. That was a solar physics joke.)

STEREO's
journey to the "dark side" began on Oct. 25, 2006,
when the twin probes left Earth together onboard a Delta II
rocket. High above the atmosphere, they separated and headed
for the Moon. What happened next was a first in space navigation.
The Moon acted as a gravitational slingshot, flinging the
two probes in opposite directions—STEREO-A ahead of Earth
and STEREO-B behind. They've been spreading apart ever since,
and this is where they are now:

Above:
The current positions of the STEREO Ahead (red) and Behind
(green) spacecraft relative to the Sun (orange) and Earth
(blue). The dotted lines show the angular displacement from
the Earth. [more]

Because
of the way the Sun spins (counterclockwise in the diagram
above), STEREO-B gets a sneak preview of sunspots and coronal
holes before they turn to face Earth—a boon for forecasters.

At
the moment, STEREO-B enjoys a 3-day look-ahead advantage over
Earth-based observatories. This has allowed researchers to
predict geomagnetic storms as much as 72 hours earlier than
ever before. On several occasions in late 2008, STEREO-B spotted
a coronal hole spewing solar wind before any other spacecraft
did. When the solar wind hit Earth, STEREO-B's long-range
forecast was validated by auroras like these:

Above:
Photographer Brian Whittaker took this picture from the window
of an airplane flying over Greenland on Nov. 9, 2008. The
auroras were sparked by a solar wind impact anticipated by
STEREO-B. Credit: Spaceweather.com.

St.
Cyr notes that experienced ham radio operators can participate
in this historic mission by helping NASA capture STEREO's
images. The busy Deep Space Network downloads data from STEREO
only three hours a day. That's plenty of time to capture all
of the previous day's data, but NASA would like to monitor
the transmissions around the clock.

"So
we're putting together a 'mini-Deep Space Network' to stay
in constant contact with STEREO," says Bill Thompson,
director of the STEREO Science Center at Goddard.

The
two spacecraft beam their data back to Earth via an X-band
radio beacon. Anyone with a 10-meter dish antenna and a suitable
receiver can pick up the signals. The data rate is low, 500
bits per second, and it takes 3 to 5 minutes to download a
complete image.

So
far, the mini-Network includes stations in the United Kingdom,
France and Japan—and Thompson is looking for more: "NASA
encourages people with X-band antennas to contact
the STEREO team. We would gladly work with them and figure
out how they can join our network."

The
two STEREO spacecraft rank among most sophisticated solar
observatories launched by NASA to date. They are equipped
with sensors that measure the speed, direction and composition
of the solar wind; receivers that pick up radio emissions
from explosions and shock waves in the sun's atmosphere; telescopes
that image the solar surface and all the tempests that rage
there; and coronagraphs to monitor events in the sun's outer
atmosphere.

"So,
really," says Guhathakurta, "we're not only seeing
the sun's dark side, we're feeling, tasting and listening
to it as well."